1. Field of the Invention
The present invention relates to an air intake system for a fuel injection engine wherein each engine cylinder is provided with a swirl inlet port and conventional straight inlet port.
2. Description of the Related Art
Engines operated on a lean air-fuel mixture having an air-fuel ratio higher than a stoichiometric ratio in the main operating range are known as lean burn engines. These lean burn engines are usually operated on a lean air-fuel mixture and are switched to operation on a rich air-fuel mixture when and acceleration or a high load operation is required.
Some learn burn engines are also equipped with swirl control valves, to thereby obtain a better combustion of the learn air-fuel mixture, and usually, these engines are provided with two inlet air passages for each engine cylinder; one leading to a swirl inlet port of the cylinder, which generates a swirl of the inlet air therethrough in the cylinder, and the other leading to conventional low pressure drop straight type inlet port.
The swirl control valve is provided in the inlet air passage of the straight port, for blocking the air passage in accordance with the load condition of the engine. For example, when the engine is operated at a low speed and low load, the swirl control valve is closed to block the inlet air passage to the straight port, and the amount of fuel injected and the ignition timing are adjusted to obtain a lean air-fuel mixture operation. When the air passage to the straight port is blocked, the major portion of the inlet air to the engine flows into the engine cylinder through the swirl inlet port, and thus a strong swirl of an air-fuel mixture is generated within the cylinder, and therefore, a stable combustion can be obtained with a lean air-fuel mixture.
When the engine is operated at a high load, high speed condition, the swirl control valve is opened to allow inlet air into the cylinder through the low pressure drop straight port, and the amount of fuel injected and the ignition timing are adjusted to obtain a rich (or stoichiometric) air-fuel ratio mixture. Accordingly, the engine output is increased due to the increased inlet air flow and richer air-fuel ratio.
In the engines equipped with swirl control valves, to widen the lean air-fuel ratio operation limits and improve the response of the engine, generally the most effective results are obtained by injecting fuel to both the swirl port and the straight port.
This type of the engine is disclosed, for example, by Japanese Unexamined Patent Publication No. 60-219454. In this engine, a fuel injection valve is installed on the respective division wall separating the inlet air passages into a swirl port and a straight port. The fuel injector is provided with two nozzles for injecting the fuel to the swirl port and the straight port, respectively, and a swirl control valve is disposed in the respective inlet air passages leading to the straight port, at a position upstream of the fuel injection nozzles.
In this type of engine, when the swirl control valve is opened, the fuel injected to the inlet air passage to the straight port is atomized by the inlet air flow through the inlet air passage.
When the swirl control valve is closed, however, the inlet air passage to the straight port is blocked, and accordingly, there is no inlet air flow for atomizing the injected fuel in the inlet air passage to the straight port. Consequently, the fuel injected to the straight port flows into the engine cylinder in an insufficiently atomized state. After combustion, these relatively large particles of fuel cause an increase in the amount of the NOx component of the exhaust gas from the engine.
To solve this problem, in the engine disclosed by above Patent Publication No. 60-219454, an air induction hole is provided in the valve element of the swirl control valve, and accordingly, when the swirl control valve is closed a small amount of air still flows into the inlet air passage leading to the straight port, through the air induction hole, and thus the fuel injected to the straight port is atomized to a certain extent by the air flow through the air induction hole.
Also, it is known that, the provision of an aperture in the division wall by which a communication is obtained between the inlet passages and the swirl port and straight port effectively improves the atomization of the fuel injected to the straight port. This aperture is disposed near the outlet of the fuel injection nozzles, and when the swirl control valve is closed, a portion of inlet air flows therethrough from the swirl port side to the straight port side, and thus the atomization of the fuel injected to the straight port is improved by this air flow.
Generally, in a lean burn operation of the engine, the amount of NOx in the exhaust gas is increased when the air-fuel ratio of the air-fuel mixture supplied to the engine is lowered (i.e., approaches the stoichiometric air-fuel ratio), and fluctuations of the engine output torque are increased when the air-fuel ratio is increased. Therefore, the air-fuel ratio (A/F) of the air-fuel mixture for a lean burn operation of the engine must be set between the lower limit (A/F) min, which gives the maximum allowable NOx emission, and the higher limit (A/F) max, which gives the maximum allowable fluctuation of the output torque of the engine. Namely, the difference between the (A/F) max and the (A/F) min defines the range of the air-fuel ratio for an allowable lean burn operation of the engine (in this specification this range, (A/F) max-(A/F) min, is called the "available area of A/F", or simply ".DELTA.A/F").
Considering variations in the engine operating conditions or deviations in the performance of individual engines due to manufacturing tolerances, preferably the available area of A/F or .DELTA.A/F is made as large as possible.
It has been found, however, that the available area of A/F, or .DELTA.A/F is greatly affected by the location and size of the air induction hole of the swirl control valve, and or the size of the aperture in the division wall.
FIG. 10 illustrates a typical arrangement of the swirl control valve and the fuel injector of lean burn engines. Referring to FIG. 10, a typical inlet air passage 114 has on curved portion 116 and a straight portion 118 connecting the curved portion 116 and a swirl port 123 of an engine cylinder 125. A swirl control valve 110 is usually disposed upstream of the curved portion 116, and fuel is injected from a nozzle 119 of a fuel injector located downstream of the swirl control valve 110. In this case, if the air induction hole of the swirl control valve 110 is provided near the inner wall of the curved portion 116 of the inlet air passage 114 (near to the wall having a smaller radius of curvature, i.e., the lower half of the valve element in FIG. 10), the air 120 from the air induction hole passes straight through the curved portion 116 and impinges on the outer wall of the curved portion 116 (i.e., the wall of the curved portion having a larger radius of curvature). Consequently, the injected fuel 112 is carried by this air flow 120 and impinges on the wall and is adhered thereto. The fuel adhered to the wall flows along the wall and enters the cylinder as a drop of liquid, and as mentioned before, this causes an increase in the amount of NOx in the exhaust gas, and thus the available area of A/F is reduced. Also, when the size of the air induction hole is large, the atomization of the injected fuel is improved due to the increased air flow through the air induction hole, but if the size of the air induction hole is too large, the amount of inlet air flowing into the cylinder through the swirl port is reduced and the swirl generated in the cylinder becomes unsatisfactorily weak. This also reduces the available area of A/F due to an insufficient mixing of the air-fuel mixture in the cylinder. Therefore, the position and size of the air induction hole are very important to the obtaining of a larger available area of A/F. Nevertheless, Japanese Unexamined Patent Publication No. 60-219454 does not teach the position and size of the air induction hole.
Further, if the aperture of the division wall is provided in addition to the air induction hole, the size of the air induction hole must be determined in conjunction with the size of the aperture.
As explained before, the atomization of the fuel injected to the straight port is effectively improved by providing an aperture in the division wall, but the size of the aperture is also an important factor in the obtaining of a larger available area of A/F. If the size of the aperture is small, the amount of the air flowing through the aperture is also small and accordingly, the fuel injected to the straight port is not sufficiently atomized, and thus the amount of NOx in the exhaust gas is increased, and the available area of A/F is reduced. Conversely, if the size of the aperture is too large, the amount of inlet air flowing into the cylinder through the swirl port is reduced, because the amount of air flowing into the cylinder through the aperture and the straight port is increased. This is a cause of an insufficient swirl in the cylinder and results in a reduction of the available area of A/F.
FIG. 8 illustrates a general relationship between the available area of A/F (.DELTA.A/F) and the area S of the aperture in the division wall. In FIG. 8, the solid line shows the change in .DELTA.A/F when the air induction hole is provided in the swirl control valve, and the dotted line shows the change in A/F when the air induction hole is not provided in the swirl control valve.
As seen from the figure, in both cases an optimum value of the area of the aperture giving a largest value of .DELTA.A/F exists, and therefore, preferably the size of the aperture is as close as possible to that optimum value. Also, as seen from FIG. 8, the .DELTA.A/F is increased when the air induction hole is provided in the swirl control valve. Namely, if the air induction hole is appropriately positioned, the air flowing there through not only improves the atomization of the injected fuel but also prevents the carrying of the injected fuel by the air flow through the aperture, which fuel will adhere to the wall of the inlet passage opposite the aperture.
As explained above, the size of the aperture in the division wall and the size of the air induction hole of the swirl control valve must be determined to be such that the maximum available area of the A/F is obtained, but usually it is difficult to obtain a desired size of the aperture in the division wall.
Generally, the intake manifold of the engine is produced by casting. Consequently, if an aperture must be formed in the division wall, it is usually formed by the use of a core during the casting, or alternatively, it is obtained by machining, for example, by drilling a hole in the cast inlet manifold. If the core is used for forming the aperture, the portion of the core forming the aperture is supported between the core forming two inlet passages of the inlet manifold, and the cross-section of the portion of the core forming the aperture must be large enough to prevent a breakage of that portion during the casting. Consequently, the size of the aperture formed by the part also becomes large; usually larger than the desired size. Namely when the aperture is formed by casting, it is difficult to obtain the optimum size of the aperture.
Alternating, as mentioned above, the aperture can be formed by drilling after the inlet manifold is cast. In this case, as shown in FIG. 9, the drill 103 must be inserted from outside of the inlet manifold 109, through a hole 101 in which the fuel injector is installed, to form the aperture 107 on the division wall 105, but due to the limited space for machining, the drilling angle .theta. shown in the figure is not large enough, and consequently, usually it is difficult to obtain an aperture having the optimum size or shape by drilling. Further, if a drilling procedure is used to machine the aperture 107, a relatively long drill is required, and this lowers the accuracy of the machining due to fluctuations of the drill chip, and causes a flash on the edge 105a of the aperture 107.